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ORIGINAL RESEARCH COMMUNICATION |
1 From the Unité du métabolisme Protéino-énergétique, National Institute for Agronomic Research/University of Auvergne, Human Nutrition Research Center Auvergne, the Centre hospitalier et Universitaire, Clermont-Ferrand, France (HD-B, LM, and YB) and and the Service de pédiatrie B, University Hospital Center, Clermont-Ferrand, France (MM)
2 Address reprint requests and correspondence to Y Boirie, Laboratoire de nutrition Humaine, BP 321, 58, rue Montalembert, 63009 Clermont-Ferrand Cedex 1, France. E-mail: yves.boirie{at}sancy.clermont.inra.fr.
| ABSTRACT |
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Objective: The aim was to establish and validate new equations for predicting resting energy expenditure (REE), specifically in obese children.
Design: REE (indirect calorimetry) and body composition (bioelectrical impedance analysis) were measured in 752 obese subjects aged 318 y. The first cohort (n = 471) was used to establish predictive equations, the second (and independent) cohort (n = 211) was used to validate these equations, and the third cohort, a follow-up group of children who lost weight (n = 70), was used to examine predictive REE in the postobese period. REE values predicted with the use of various published equations and the new established equation were compared with measured REE by using the Bland-Altman method and Students t tests.
Results: In cohort 1, significant determinants of the new prediction equations were fat-free mass in boys (model R2 = 0.79) and age and fat-free mass in girls (model R2 = 0.76). External validation conducted by using the Bland-Altman method and Students t tests, in cohort 2, showed no significant difference between measured REE and predicted REE with the new equation. When already published equations were applied, systematical bias appeared with all published equations except for that of the World Health Organization. In cohort 3, the children who lost weight, almost all equations significantly underestimated REE.
Conclusions: These new predictive equations allow clinicians to estimate REE in an obese pediatric population with sufficient and acceptable accuracy. This estimation may be a strong basis for energy recommendations in childhood obesity.
Key Words: Resting energy expenditure body composition obesity fat-free mass predictive equation children
| INTRODUCTION |
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Equations to predict REE in obese subjects, especially in children, have already been established (10). Although FFM explains interindividual variations in REE better than does body weight (11, 12), body composition is rarely considered, particularly in children. Moreover, the applicability of these equations and their accuracy have not been tested in the same population after changes in body composition, eg, weight loss.
Therefore, the 3 aims of the first part of this study were to establish new predictive equations using body composition, on a large sample of French obese children, to validate the equations on a second independent population of obese children, and to evaluate their accuracy in a longitudinal follow-up survey in which body composition and REE were simultaneously measured. Finally, boys and girls with a large age range, 318 y old, were included to allow examination of a period of life that is characterized by various changes in growth rate and metabolic changes. In a second part, the accuracy of the newly established equations was compared with that of published equations: a World Health Organization (WHO) equation (6), the Harris-Benedict equation (7), the Schofield weight and height equations (8), and the equation of Tverskaya et al (Tverskaya equation; 10).
| SUBJECTS AND METHODS |
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2 and must be visiting a pediatric nutritionist for the first time. Children and parents must agree to nutritional follow-up with weight control, nutritional and exercise advice, and REE measurement. For cohort 1, the inclusion period was the years 1993 through 1999, and, for cohort 2, it was the period from 1 January 2001 to 1 March 2002. REE was measured at the Human Nutrition Laboratory (Clermont-Ferrand, France) after exclusion of any evolving disease. A total of 471 children constituted the cohort 1. New equations were established in this population. We validated these equations in cohort 2 (n = 211), which was constituted of other obese children who were referred between 1 January 2001 and 1 March 2002 for an REE measurement.
Some of the cohort 1 children were measured again after the first investigation; 70 children had lost weight (
± SD weight loss: 12.08 ± 6.94%). This population, considered cohort 3 in this study, allowed testing of the validity of newly established equations when weight changes occurred.
| Methods |
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The SD score for BMI (z score) was determined by the following formula:
![]() | (1) |
Assessment of body composition
The assessment consisted of whole-body BIA and measurements of height and weight, from which BMI was calculated. While wearing their underwear, patients were weighed on a mechanical scale that was precise to within 0.1 kg (709; SECA, Hamburg, Germany). Height was measured to 0.1 cm with a height gauge that was accurate to within 0.2 cm. BIA (101; RJL System, Detroit) was performed in the postabsorptive state, after 10 min of rest in a supine position and while the patients had an empty bladder. The algorithms used were validated in a population of obese children and adolescents by Wabitsch et al (14). These measurements were always performed by the same investigator (LM).
Energy expenditure measurement
An open-circuit indirect calorimetry procedure was performed for
45 min by using a Deltatrac II apparatus (Datex Engström, Helsinki). Before each test, the gas analyzers were calibrated with a reference gas mixture (95.0% O2 and 5.0% CO2). REE was measured after an overnight fast. The subjects were monitored during the measurements to prevent any movement or sleeping under the hood. The first 10 min of each study was excluded to account for environmental adjustment by the children and gas adaptation in the hood. Then REE was calculated from oxygen consumption and carbon dioxide production by using the equation of De Weir (15).
Predicted equations
We compared new equations with published equations. Thus, REE values predicted with the use of a new equation and published equations were compared with measured REE. The WHO (6), Harris-Benedict (7), and Schofield (8) equationswhich are not specifically dedicated to obese populationsand the Tverskaya equation (10) involving obese children were used in this study. These equations are shown in Table 1
for the sake of clarity.
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The aim of the first part was to establish and validate new equations in a large cohort of French children. In the first population (cohort 1), statistical analyses were performed by simple linear regression on variables related to measured REE. Multivariate analyses were conducted by using multiple linear regression and integrating all factors for which P value in the simple linear regression was
0.20. The threshold for significance was set at 0.05 for all statistical analyses. This multivariate analysis enabled us to obtain our own equation by using a stepwise selection. In each step, R2, SE, and likelihood ratio were analyzed. These factors were introduced in a final model to predict REE. The Bland-Altman method (16) permitted us to plot the difference between measured REE and predicted REE with a new equation, and then Students t tests were used to underline a systematical bias. To validate the new equation in cohort 2, the slope and the intercept of simple linear regression between measured REE and predicted REE with new equation were compared with 1 and 0, respectively. Finally, we analyzed the accuracy of the new equation in a population of subjects who lost weight (cohort 3) by comparing that equation with the published equations by using the Bland-Altman plot and Students t tests. When a significant difference was found, we applied a multivariate analysis with a stepwise procedure in the population who lost weight to ascertain which factors better explained REE in this population.
The second part of analysis aimed to compare the accuracy of predicting REE with the new equations and the published equations. Multiple comparisons using Dunnetts test permitted the testing of differences between measured and predicted REE obtained with the Harris-Benedict, WHO, Tverskaya and Schofield equations. The Bland-Altman plot (16) and Students t tests were used to assess agreement between predicting equations.
| RESULTS |
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![]() | (2) |
![]() | (3) |
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In cohort 3, those subjects who lost weight (12% of initial weight and 5% of initial fat mass), all predictive equations overestimated REE by 0.70% to 7.45% (Table 5
). With the use of the Bland-Altman method and Students t tests, all predictive equations except the Tverskaya equation misestimated REE (P = 0.80).
| DISCUSSION |
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In the new equations, as expected, FFM was the major determinant of REE; it explained 79% and 76% of REE in boys and girls, respectively,. These results are in agreement with other reports (12). In boys, the new equation was the most accurate for use in the obese population, but, in girls, age was also a significant determinant of REE. This finding suggests a greater influence of hormonal factors in girls: our sample included children aged 318 y, which included those in the pubertal period. This period is associated with rapid anatomical and physiologic changes, including variations in metabolic rate and energy requirements. REE in obese children and adolescents was significantly higher than that in normal-weight children (20). When REE was adjusted for body-composition differences, no differences persisted between obese and nonobese populations in some reports (21), but they did persist in others (22).
When an external validation was conducted in cohort 2, no significant difference between measured and predicted REE with the newly established equation was found. REE was well predicted by the new equation. The Bland-Altman method allowed us to consider the new equation as a good tool for predicting REE. When we tested the simple linear regression (Figure 1
), the slope was significantly different from 1, and the intercept was significantly different from 0. There was effectively a negative bias, which was strongly linked to the z score, and there was more inaccuracy in a high z score. Therefore, we rebuilt the model with consideration of the z score, but R2 was not improved (0.75). This problem was observed recently in extremely obese adult women and reported (23). Indeed, the authors of that report stated that, in studies in the morbidly obese, predictive equations developed for nonobese populations were more accurate (3%) than were the obese-specific equations, because REE can be 40% overpredicted or 21% underpredicted by the equations developed for use in moderately obese populations. Therefore, we recommend caution when using predictive equations in extremely obese children. When a weight loss occurred in these subjects, both new and published equations, except the Tverskaya equation, significantly overestimated REE. With multiple linear regression, relative FFM loss explained 6% of measured REE variability in boys. Thus, we believe that all these equations are to be used only during the weight-stable period and not during the weight-changing period. The equations should be applied in obese children before any weight loss or after weight stabilization in the postobesity state. Maintenance of a reduced body weight is associated with compensatory changes in EE that oppose the maintenance of a body weight that is different from the usual weight (24, 25). Predictive equations must be established in a weight-stable period when energy metabolism has adapted.
All predicted equations used in this study led to significant miscalculations of REE. Individual estimations were markedly underestimated (
31%) or overestimated (57%). Previous studies established various equations to measure REE by using BIA. With restriction to the analysis of obese children, the WHO equation overestimated measured REE in 4 studies (10, 2628) but not in the fifth study (29). These reports also concluded that differences existed according to sex (26, 27). The question now is to justify the choice of one formula rather than another. Individual underestimations or overestimations might have a deleterious effect on diet prescription and may limit weight loss. These calculations misestimated REE by 3.76 MJ/d, which is not acceptable in clinical practice. This underestimation could lead to a miscalculation in dietary recommendations. The populations used for validation can explain these large errors. Harris and Benedict derived their equations by using data from healthy nonobese children (7), so these formulas were not specific to obese children. In agreement with literature, all predictive equations underestimated or overestimated REE according to the population (10). Predictive equations were population specific and constituted a population-based REE estimation, which is less valid on an individual scale. So the question remains as to the extent to which such a variation between individual REE measurements and REE from predictive equation should be tolerated. Predictive equations permit a first estimation of REE. When a failure of dietetic intervention occurred, REE measurement was appropriate.
Body composition was estimated by BIA. This is a useful technique for body-composition analysis in healthy persons who are overweight or mildly to moderately obese (30, 31). Among numerous studies, Utter et al (31) compared body composition evaluated by leg-to-leg BIA or underwater weighing before and after a weight-reduction program in obese and normal-weight control subjects. No significant difference was found between underwater weighing and BIA in estimating the FFM at the baseline and after weight loss. However, BIA values are affected by numerous variables including body position, hydratation status, ambient air, skin temperature, and recent physical activity (32). Equations used by various BIA apparatuses are different and not always known with precision. BIA validity has not really been confirmed, particularly in severely obese children (33, 9). A well-defined procedure specifically for performing routine BIA measurements and the equations used for BIA are necessary (30). In this study, external validation in an independent population of obese children confirmed the validity of the new equation. Relative FFM loss must be taken into account when obese children lose weight, but metabolic adaptation during rapid weight modifications should also be considered (25). In conclusion, new predictive equations for REE calculations in obese children have been validated in a large population with an accuracy sufficient to allow clinicians to better estimate energy balance in pediatric subjects. These equations have been compared with other equations not specifically dedicated to obese subjects of all ages or developed for pediatric populations, particularly obese and formerly obese children. Because body-weight changes are associated with modifications in the relation between lean mass and metabolic rate, it is recommended that these equations are used in a several-week period of body-weight stability. When REE is measured, these equations may be helpful in detecting specific alterations in the regulation of energy balance that lead to severe obesity in children.
| ACKNOWLEDGMENTS |
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HDB contributed to the design of the study, data collection, data analysis and interpretation, and manuscript writing. MM enrolled the children in the study and contributed to the manuscript preparation. LM was responsible for all measurements and data collection. YB was responsible for study conception, data interpretation, and manuscript preparation. None of the authors had any financial or personal conflicts of interest.
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